1. Field of the Invention
The present invention is related generally to implantable medical electrical leads. More specifically, the present invention is related to implantable neurological leads.
2. Prior Art
Spinal cord and other neurological stimulations by electrical leads are used for many purposes, including pain masking. These electrical leads emit a voltage or current which mimics the body's electrical response and masks a patient's pain.
As such, these leads must be precisely placed near a specific nerve or series of nerves to provide the required therapy. It is therefore critical that the correct nerve or nerves are targeted and that the lead does not move once in place. Lead migration, defined as the undesirable movement of a lead in the body over time, causes the stimulation therapy to become ineffective. Additional surgery is therefore required to reposition the lead and correct the problem.
One such neurological electrical stimulation lead is the percutaneous lead, which is well suited for stimulating nerve tissue. A percutaneous lead is a relatively long, slender, cylindrical lead with a small diameter and a series of electrode bands that wrap around the outside surface. The relatively long length and small cylindrical diameter allow for easy and unimpeded access to the nerve tissue along the spinal column through a small incision in the body. The series of electrode bands are programmed to emit an electrical signal such as a voltage or current that provides pain relief by targeting a specific nerve or nerves. However, the long, slender, cylindrical construction of percutaneous leads which make them advantageous for intricate placement and unimpeded advancement about the tight confines of the spinal column also make them prone to movement and migration.
One such solution to prevent lead migration is a suture type of lead fixation device. As discussed in U.S. Pat. Nos. 5,843,146 and 6,473,654 to Cross and Chinn, respectively, implanted leads are secured through the use of sutures designed to tie the implanted lead to bodily tissue.
The problem is that the use of sutures is not ideal in securing the lead to delicate neurological tissue, which may easily tear. Furthermore, suturing neurological stimulation leads requires invasive surgery to gain access to the spinal column area, which would defeat the minimally invasive benefits of the percutaneous lead. In addition, sutures can make it difficult to easily move the stimulation lead to a new desired location. A physician would have to perform another invasive surgery to gain access to the spinal column area to remove the old sutures and re-suture the lead to the new location.
Helical screw anchoring mechanisms are another means of fixating implanted leads. Such mechanisms are disclosed in U.S. Pat. No. 6,711,443 and U.S. Patent application publication 2007/0299493, both to Osypka. Helical fixation mechanisms are primarily used in the placement of cardiac leads. This fixation mechanism is beneficial in cardiac applications since the strong fibrous tissue of the heart muscle captures and prevents the embedded helical structure from becoming unsecured.
Helical fixation mechanisms, however, are not ideal in securing a lead to neurological tissue. The drilling action of the helix destroys the delicate neurological tissue as it bores into the tissue. In addition, since neurological tissue is not as strong and fibrous as that of cardiac tissue, the helical structure would easily rip out of and damage the delicate neurological tissue.
Hook style lead anchoring mechanisms have also been developed to secure cardiac leads to heart tissue. Two previous hook type anchoring mechanisms have been disclosed in U.S. Pat. Nos. 4,858,623 and 5,871,532 by Bradshaw et al. and Schroeppel, respectively. However these specific prior art anchoring examples are not well suited for anchoring to delicate neurological tissue. They could easily be dislodged through movement as a patient goes about their daily activities. As stated in column 4, line 65 of the '623 patent, “a slight tug on the lead will cause the hook to rotate about its pivot point to a position beyond the tip, thereby allowing the lead to be withdrawn”. Such inadvertent dislodgement could occur through the movement of a patient's spinal column as they move.
Accordingly, what is desired is a percutaneous neurostimulator lead with a fixation mechanism that provides long term secure anchoring, prevents inadvertent lead dislodgement, minimizes neurological tissue damage and does not compromise the minimally invasive benefits of the lead.